Switching shading mode to rendered doesn't switch to external styles for new viewport areas

[Andrej730]

Creating this just to keep the track of this issue.

In c7b3331 I've added support for automatically switching to external styles if shading mode is set to "RENDERED" but currently it doesn't work for viewports that were added during current blender session or if some area was converted to viewport. Restart helps though.
Not sure how it can be done - need to find some way in Blender Python API to detect newly created viewports.